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Don't copy executable code into a CodeMemory (#3265)

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* Don't copy executable code into a `CodeMemory`

This commit moves a copy from compiled artifacts into a `CodeMemory`. In
general this commit drastically changes the meaning of a `CodeMemory`.
Previously it was an iteratively-pushed-on structure that would
accumulate executable code over time. Afterwards, however, it's a
manager for an `MmapVec` which updates the permissions on text section
to ensure that the pages are executable.

By taking ownership of an `MmapVec` within a `CodeMemory` there's no
need to copy any data around, which means that the `.text` section in
the ELF image produced by Wasmtime is usable as-is after placement in
memory and relocations have been resolved. This moves Wasmtime one step
closer to being able to directly use a module after it's `mmap`'d into
memory, optimizing when a module is loaded.

* Fix windows section alignment

* Review comments
This commit is contained in:
Alex Crichton
2021-08-30 13:38:35 -05:00
committed by GitHub
parent eb251deca9
commit ef3ec594ce
8 changed files with 215 additions and 236 deletions
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25
crates/jit/src/link.rs
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@@ -1,6 +1,6 @@
//! Linking for JIT-compiled code.
use object::read::{Object, ObjectSection, Relocation, RelocationTarget};
use object::read::{Object, Relocation, RelocationTarget};
use object::{elf, File, NativeEndian as NE, ObjectSymbol, RelocationEncoding, RelocationKind};
use std::convert::TryFrom;
use wasmtime_runtime::libcalls;
@@ -9,24 +9,13 @@ type U32 = object::U32Bytes<NE>;
type I32 = object::I32Bytes<NE>;
type U64 = object::U64Bytes<NE>;
/// Links a module that has been compiled with `compiled_module` in `wasmtime-environ`.
/// Applies the relocation `r` at `offset` within `code`, according to the
/// symbols found in `obj`.
///
/// Performs all required relocations inside the function code, provided the necessary metadata.
/// The relocations data provided in the object file, see object.rs for details.
///
/// Currently, the produced ELF image can be trusted.
/// TODO refactor logic to remove panics and add defensive code the image data
/// becomes untrusted.
pub fn link_module(obj: &File, code_range: &mut [u8]) {
// Read the ".text" section and process its relocations.
let text_section = obj.section_by_name(".text").unwrap();
for (offset, r) in text_section.relocations() {
apply_reloc(obj, code_range, offset, r);
}
}
fn apply_reloc(obj: &File, code: &mut [u8], offset: u64, r: Relocation) {
/// This method is used at runtime to resolve relocations in ELF images,
/// typically with respect to where the memory was placed in the final address
/// in memory.
pub fn apply_reloc(obj: &File, code: &mut [u8], offset: u64, r: Relocation) {
let target_func_address: usize = match r.target() {
RelocationTarget::Symbol(i) => {
// Processing relocation target is a named symbols that is compiled